1. Field of the Invention
The present invention relates to a differential output circuit using current mirror circuits in a differential manner and a communication device using such a differential output circuit.
2. Description of the Related Art
FIG. 1 is a diagram illustrating an exemplary configuration of a general differential output circuit that uses current mirror circuits in a differential manner.
As shown in FIG. 1, the differential output circuit 1 includes p-channel MOS (PMOS) transistors Q1 to Q4, current sources I1 and I2, a bias power supply VB, and load resistors Rload1 and Rload2. In FIG. 1, C1 and C2 represent capacitors, respectively.
Among these structural components, a first current mirror circuit CM1 is constructed of the PMOS transistors Q1 and Q2, the current source Il, and the load resistor Rloadl, and the capacitor Cl. In contrast, a second current mirror circuit CM2 is constructed of the PMOS transistors Q3 and Q4, the current source 12, the load resistor Rload2, and the capacitor C2. In each of the PMOS transistors Q1 and Q2, the gate and the drain thereof are diode-connected to each other.
In the differential output circuit 1, a voltage change in the gate G1 of the transistor Q1 when increasing or decreasing the master current IinP of the first current mirror circuit CM1 can be determined by the diode differential resistance of the transistor Q1 and the capacitor C1 of gate G1. For example, since a diode dynamic resistance is small in a state of being biased with a large current, the gate voltage changes quickly in such a state and the output of the slave transistor Q2 then changes quickly. Conversely, the diode differential resistance is high in a state of being biased with a small current, so that both a gate potential and a slave current will change slowly.